Most current approaches to concurrency control in database systems rely on locking of data objects as a control mechanism. In this paper, two families of nonlocking concurrency controls are presented. The methods used are “optimistic ” in the sense that they rely mainly on transaction backup as a control mechanism, “hoping ” that conflicts between transactions will not occur. Applications for which these methods should be more efficient than locking are discussed.

A sequence of interleaved user transactions in a database system may not be 8zis, i.e., equivalent to some sequential execution of the individual transactions. Using a simle transaction model we show that recognizing the transaction histories which are serializable is an NP- complete problem. We therefore introduce several efficiently recognizable j9gsses of the class of serializable histories most of thee S.b- classes correspond to serializability principles existing in the literature and used in practice. We also propose two new principles which subsume all previously known ones. We give necessary and sufficient conditions for a class of histories to be the output of an efficient history scheduler these conditions imply that there can be no efficient scheduler that outputs all of serializable histories,.and also that all subclasses of serializable histories studied above have an efficient scheduler. Finally, we show how our results can be extended to far more general transaction models,'to transactions with partly interpreted functions, and to distributed database syst%.

When the only information available about transactions is syntactic information, serializability is the main correctness criterion for concurrency control. Serializability requires that the execution of each transaction must appear to every other transaction as a single atomic step (i.e., the execution of the transaction cannot be interrupted by other transactions). Many researchers, however, have realized that this requirement is unnecessarily strong for many applications and can significantly increase transaction response time. To overcome this problem, a new approach for controlling concurrency that exploits the semantic information available about transactions to allow controlled nonserializable interleavings has recently been proposed. This approach is useful when the cost of producing only serializable interleavings is unacceptably high. The main drawback of the approach is the extra overhead incurred by utilizing the semantic information. We examine this new approach in this paper and discuss its strengths and weaknesses. We introduce a new formalization for the concurrency control problem when semantic information is available about the transactions. This semantic information takes the form of transaction types, transaction steps, and transaction break-points. We define a new class of “safe ” schedules called relatively consistent (RC) schedules. This class contains serializable as well as nonserializable schedules. We prove that the execution of an RC schedule cannot violate consistency and propose a new concurrency control mechanism that produces only RC schedules. Our mechanism assumes fewer restrictions on the interleavings among transac-tions than previously introduced semantic-based mechanisms.

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The semantics of objects and transactions in database systems are investigated. User-defined predicates called consistency assertions are used to specify user programs. Three new correctness criteria are proposed. The first correctness criterion consistency is based solely on the users’ specifications and admits nonsenalizable executions that are acceptable to the users. Integrity constraints of the database are maintained through consistency assertions. The second correct-ness criterion orderabdlty is a generalization of view serializability and represents a weak notion of eqmvalence to a serial schedule. Finally, the third correctness criterion strong order-abzlity is introduced as a generalization of conflict serializabihty. Unlike consistency, the notions of orderability allow users to operate in isolation as maintenance of the integrity constraints now becomes the responsibility of the database system,

Current commercial databases allow application programmers to trade off consistency for performance. However, existing definitions of weak consistency levels are either imprecise or they disallow efficient implementation techniques such as optimism. Ruling out these techniques is especially unfortunate because commercial databases support optimistic mechanisms. Furthermore, optimism is likely to be the implementation technique of choice in the geographically distributed and mobile systems of the future. This thesis presents the first implementation-independent specifications of existing ANSI isolation levels and a number of levels that are widely used in commercial systems, e.g., Cursor Stability, Snapshot Isolation. It also specifies a variety of guarantees for predicate-based operations in an implementation-independent manner. Two new levels are defined that provide useful consistency guarantees to application writers; one is the weakest level that ensures consistent reads, while the other captures some useful consistency properties provided by pessimistic implementations. We

Real-time systems (RTS) respond to their environment within specified time constraints. RTS are inherently concurrent and typically manage shared data resources, so they require synchronization to ensure both logical and timing correctness. Much research in managing shared data has been carried out in the context of database systems. It is therefore appropriate to ask, "What principles and techniques can RTS borrow from database concurrency control?" Concurrency control techniques can seldom be transferred from database to the real-time domain without change; the performance considerations are too different. We discuss common features and differences between the two domains, paying special attention to the assumptions and goals of different classes of real-time systems. We then survey an array of correctness criteria and concurrency control techniques with regard to their applicability in soft, firm, and hard-deadline RTS. Real-time scheduling techniques and their interaction with conc...

Semantics-Based Optimization Under Epsilon Serializability Wenwey Hseush The recent movement to advanced applications and the continuous drop in hardware prices impact the design of on-line transaction processing (OLTP). The data contention problems imposed by serializability become an increasingly serious performance bottleneck. Many efforts have been made to decrease data contention in OLTP systems. In general, two directions of work have been studied. The first direction (semantics-based optimization) is to design practical optimization techniques while preserving serializability as the correctness criterion. The second direction is to relax the restrictions on serializability. A good example is the design of epsilon transactions that operate under the semantics of epsilon serializability (ESR). The goal of this thesis is to study the relationship between ESR and semantics-based optimization and design new methods to further improve performance. We show that ESR and several well...

The most commonly used model for concurrency control in traditional database systems represents transactions as streams of partially ordered operations that are scheduled for execution by a central or distributed transaction manager. The various scheduling strategies are proved correct by using the serializability theory. This theory, in its classical form, operates successfully on systems with at (non-nested) transactions but is unable to represent conveniently complex (nested) computations, inherent to the object-oriented paradigm.